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Rust email SDK

The Rust email crate: receive, read and send mail from Rust.

Receive email in Rust without running a mail server. Create an address, wait for the message that lands on it, read the text, HTML and attachments, and reply in the same thread. Mail is stored encrypted and opened in your own process, with typed results and errors.

Install the Rust crate
cargo add mailcycle --features events
Create a free accountRead the Rust guide

mailcycle on crates.io · Rust 1.75 or later · Apache 2.0 · Free tier, no card

Quickstart

Receive and send email in Rust.

Sign in, create an address, wait for a message and answer it. That is the whole loop, and it is the same loop whether the mail is a sign-up code, a customer or a receipt.

  1. 1Install mailcycle and set MAILCYCLE_PHRASE to your account's recovery phrase.
  2. 2Create an address with mc.addresses().create(). It receives mail straight away.
  3. 3Wait for the next message with mc.messages().wait_for(), or take it from a webhook.
  4. 4Reply with mc.messages().send(), threaded under the original.
The full Rust guide
Rust: create an address, wait for mail, reply
use mailcycle::{Mailcycle, NewAddress, Outgoing, WaitFor};

let mut mc = Mailcycle::sign_in(&std::env::var("MAILCYCLE_PHRASE")?, Default::default())?;

// A new address that receives mail, sealed to your key.
let address = mc.addresses().create(NewAddress {
    prefix: Some("signup".into()),
    label: Some("Test run 42".into()),
    ..Default::default()
})?;
println!("{}", address.email_address); // signup-x7k2@...

// Wait for the verification email, decrypted here.
let message = mc.messages().wait_for(WaitFor {
    address_id: Some(&address.id),
    subject: Some("verification"),
    ..Default::default()
})?;
println!("{} {}", message.subject, message.text);

// Reply in the same thread.
mc.messages().send(Outgoing {
    reply_to: Some(&message),
    ..Outgoing::new(&address.email_address, &message.reply_to, "Re: Hello", "Thanks, confirmed.")
})?;
What it does

The whole email API, from Rust.

Everything the Mailcycle console can do is in the crate: addresses, inbound mail, sending, webhooks, events, API keys and devices.

Create email addresses from Rust

mc.addresses().create() mints a new address on a shared domain, or on your own domain from Scale, and it receives mail at once. Give it a prefix and a label sealed on your machine.

Receive email and wait for it

mc.messages().wait_for() listens on the event stream and returns the next matching message, filtered by sender or subject, opened and ready to read. Without a stream it checks every five seconds.

Read and parse incoming email

mc.messages().all() and .attachment() return the sender, subject, plain text, the original HTML, a safe HTML with remote images and trackers removed, and every attachment as bytes.

Send email and reply in the thread

mc.messages().send() sends from an address your account owns, with up to ten files. Pass the message you are answering and the threading headers are filled in.

Inbound email webhooks

Register a URL and every message.received event is POSTed to it, signed with HMAC-SHA256 and retried. verify_webhook_signature_now() checks the signature in one line.

A live event stream

mc.events().stream() carries the same events over a WebSocket the moment they happen, and reconnects after a drop. Events carry ids, never content.

Email testing

Test email verification in cargo test and fantoccini.

Create an address in an integration test, drive the sign-up flow, then call wait_for and read the code from message.text. A timeout is an Error::Api with code "timeout", so the ? operator fails the test with a reason.

  • A real address on a real domain, so the mail goes through the same path your users' does.
  • One call blocks until the message lands, filtered by subject or sender, with a timeout you set.
  • Text, HTML and attachments come back parsed, so pulling out a code or a link is a regular expression.
  • Mail received from the moment you start waiting counts, even if it lands before the stream is up.
Inbound email webhooks

Handle incoming email in Axum, Actix Web and Rocket.

Rather than polling, let the mail come to you. A webhook POSTs each event to your URL the moment it happens, signed so you can prove it came from Mailcycle, and retried if your server is down.

Events carry ids, never content. Fetch the message by id and it is opened in your code, exactly as it would be otherwise. Webhooks are on Operator and above.

Webhooks in the docs
Axum: verify a webhook before trusting it
use axum::{body::Bytes, http::{HeaderMap, StatusCode}};
use mailcycle::verify_webhook_signature_now;

async fn mailcycle_webhook(headers: HeaderMap, body: Bytes) -> StatusCode {
    let signature = headers
        .get("Mailcycle-Signature")
        .and_then(|value| value.to_str().ok());
    // The body exactly as it arrived, before any parsing.
    if !verify_webhook_signature_now(&secret(), &body, signature) {
        return StatusCode::UNAUTHORIZED;
    }

    let event: serde_json::Value = serde_json::from_slice(&body).unwrap_or_default();
    if event["type"] == "message.received" {
        queue(event["payload"]["messageId"].as_str().unwrap_or_default());
    }
    StatusCode::NO_CONTENT
}
No mail server

Instead of lettre plus the imap crate.

lettre sends mail through an SMTP transport you supply, and reading it back means the imap crate, a mailbox with a password, and a MIME parser such as mail-parser.

This crate does both over HTTPS. An address is a struct, mail arrives on a webhook or through wait_for, and messages come back parsed, with safe_html and attachments as Vec<u8>. The encryption is checked against Mailcycle's published test vectors.

It is not a bulk sender. If you need newsletters or high volumes of transactional mail, use a sending service for that; Mailcycle is for addresses your code owns and the mail that comes back to them.

End-to-end encrypted

Mail is opened in your Rust code, not on our servers.

Each address has a key pair derived from your recovery phrase. Incoming mail is sealed to its public key before anything is written down, and the crate derives the private key on your machine to open it.

How the encryption works
  • Signing in proves you hold the key by answering a challenge, so the phrase never crosses the wire.
  • Bodies, attachments, address labels and API key names are all sealed. The server stores ciphertext.
  • Mail sealed by one SDK opens in every other, and in the app, because they share one published format.
  • The crate is checked against Mailcycle's published test vectors, including the cases that must be refused.
  • Because the server cannot read mail, there is no server-side search. Search runs where the mail is opened.
Use cases

What developers build with the Rust SDK.

It runs in web services, background workers, CLIs and Linux devices: anywhere Rust code needs an email address of its own.

End-to-end tests for sign-up and login

Verification emails, one-time passcodes, magic links, password resets and invitations, checked by a real test in cargo test and fantoccini.

A support or orders inbox your code answers

Give an address to a queue, read what customers send, and reply from the address they wrote to, threaded under their message.

Receipts, invoices and alerts as data

Point suppliers, monitoring and other services at an address and pull the text and attachments straight into your own records.

An address per customer, job or device

Mint addresses on demand and keep each stream of mail separate, with its own label, and delete one when the job is done.

Building an AI agent? It can have an inbox too, through the Mailcycle MCP server, without writing any Rust at all.

The package

Rust SDK at a glance.

Maintained by Mailcycle, the team that runs the API.

The Mailcycle Rust SDK at a glance
Packagemailcycle
Registrycrates.io
Install withcargo add
NeedsRust 1.75 or later
AlsoThe events feature adds the live event stream; without it, wait_for checks every five seconds.
LicenceApache 2.0, free to use in your own applications
APIJSON over HTTPS at api.mailcycle.email
GuideThe Rust guide in the docs

What each plan unlocks. Free receives mail. A signed-in session sends from Personal up. API keys and webhooks start at Operator, and sending from an API key at Scale. See pricing.

Questions

Rust email SDK questions.

Add the crate with cargo add mailcycle, sign in with Mailcycle::sign_in, and create an address with mc.addresses().create(). Mail sent to it is stored encrypted, and mc.messages().all() or mc.messages().wait_for() returns it decrypted and parsed. No SMTP server and no IMAP.

Call mc.messages().send(Outgoing::new(from, to, subject, text)) with an address your account owns. Set reply_to to the message you are answering and the threading headers are filled in. Sending from a signed-in session works from the Personal plan, and from an API key on Scale and above.

Rust 1.75 or later. Enable the events feature for the live event stream; without it, wait_for checks the address every five seconds instead.

The crate is free and open source under the Apache 2.0 licence. The Mailcycle Free plan costs nothing and needs no card: 3 email addresses and 3,000 messages received a month, receive only. Sending starts on Personal at $7 a month, API keys and webhooks on Operator at $15, and sending from an API key on Scale at $19.

No. Incoming mail is sealed to the address's public key before it is stored, and the private key is derived from your recovery phrase inside your Rust process. The server holds ciphertext it cannot open. Mail you send leaves in plain text so the recipient's server can read it, and the copy kept in Sent is sealed like the rest.

Call mc.addresses().create() at the start of the run and delete the address at the end, which frees its place on your plan. Each address receives mail as soon as it exists. This is for testing your own product and for mail meant for you: registering accounts on other services in bulk, or against their terms, breaks the Acceptable Use Policy.

No. Mailcycle is inbound email infrastructure, and sending is there to answer what arrives. Sending is capped at 60 messages an hour per account and 20 an hour per device or agent, with a daily limit set by the plan, and the Acceptable Use Policy rules out bulk, marketing and outreach mail.

The API is plain JSON over HTTPS at api.mailcycle.email, so anything can call it. Message content comes back encrypted, though, and the crate is what derives the keys and opens it. Without an SDK you would implement the published encryption format yourself.
Other languages

The same API in every language you use.

Nine SDKs share one encryption format, so a message created by one opens in all the others.

Give your Rust code an email address.

Create an account, create an address, and read what arrives. The free tier needs no card.

Get startedRead the Rust guide